Although enzalutamide and abiraterone are effective initially for the treatment of castration-resistant prostate cancer (CRPC), resistance to both drugs occurs frequently through the mechanisms which are incompletely understood. Considerable evidence from both clinical and experimental studies demonstrated that androgen receptor splice variant AR-V7 plays vital roles in promoting CRPC progression and induction of resistance to enzalutamide and abiraterone. AR-V7 is not targeted by either enzalutamide or abiraterone and therefore, there is an urgent need to develop novel treatment strategies that target AR-V7 to overcome resistance. Currently, no AR-V7 targeting agents have shown clinical efficacy for CRPC despite convincing experimental and clinical data supporting a clear role for AR-V7 in resistant CRPC. We previously identified that niclosamide, an anthelmintic agent approved by the FDA for the treatment of tapeworm infections, inhibits expression of AR variants such as AR-V7 and overcomes resistance to enzalutamide and abiraterone in preclinical CRPC models. To improve the bioavailability and potency of niclosamide, we synthesized a library of niclosamide analogs according to predicted bioavailability. We have identified several novel small molecule inhibitors of AR-V7 (ARVib). We showed that ARVib has better efficacy in inhibition of AR-V7 expression, AR- V7 mediated transcriptional activity, and anti-CRPC tumor growth than niclosamide. Furthermore, we demonstrated that ARVib synergizes with enzalutamide/abiraterone. This proposal is aimed to functionally characterize and validate the most promising AR variant inhibitors (ARVib) and develop next generation treatment strategies for resistant CRPC by targeting AR-V7 using ARVib. The overall hypothesis is that inhibition of AR variants by ARVib suppresses CRPC tumor growth and overcomes resistance to improve enzalutamide/abiraterone therapy. In Aims 1 and 2, we will characterize and optimize the identified selective inhibitors of AR variants (ARVib), and evaluate the efficacy of these ARVib for their anti-tumor activity and their ability to sensitize resistant cells to enzalutamide/abiraterone treatment in resistant CRPC and PDX models in vitro and in vivo. Aim 3 will determine the mechanisms of action (MOA) of the ARVib in anti-tumor activity and resensitization to enzalutamide/abiraterone.